Possibility of Al-Si Brazing Alloys for Industrial ...
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J. Microelectron. Packag. Soc., 24(3), 35-40 (2017) https://doi.org/10.6117/kmeps.2017.24.3.035
Print ISSN 1226-9360 Online ISSN 2287-7525
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Possibility of Al-Si Brazing Alloys for Industrial Microjoining Applications
Ashutosh Sharma, and Jae Pil Jung†
Dept. of Materials Science and Engineering, University of Seoul, 163, Seoulsiripdae-ro, Dongdaemun-gu, Seoul 02504, Korea
(Received September 8, 2017: Corrected September 19, 2017: Accepted September 21, 2017)
Abstract: Aluminium alloys have been used widely since hundreds of years in automotive joining. Silicon is an excellent
alloying element that increases the fluidity, depresses the melting temperature and prevents shrinkage defects during solid-
ification, and is cost effective raw material. In recent few decades, research on cast Al–Si alloys has been expanding glob-
ally in military, automobile and aerospace industries. These alloys are good wear and corrosion resistant which depends
on processing parameters and service conditions. However, the formation of big Si-needles in Al-Si alloys is a serious
issue in joining industries. Silicon modification treatments are generally carried out to improve their durability and strength.
This paper covers an elaborative study of various Al-Si alloys, the modification strategies to refine the Si-needles, effect
of processing parameters and joining characteristics for automotive applications.
Keywords: Al alloys, Brazing, Welding, Silicon, Casting, Automotive
1. Introduction
1.1. Alloy
An alloy can be defined as a material having metallic
properties as a result of a combination of two or more elements
where at least one element is a metal. The metal atoms are
predominant in chemical composition and the metallic
bonding.1-2) In general, the alloys have a wide range of
properties different from the individual elements. Alloy is
generally synthesized by mixing one or more metals or
nonmetals at an atomic scale to enhance the various
thermodynamic and mechanical properties. As a simple
example, the alloy of Fe and C is steel, which is stronger than
each of Fe and C primary elements. The physical properties
of these alloys, like density and conductivity, may or may not
differ greatly from the individual components. However,
there are certain advanced engineering characteristics, e.g.,
tensile strength, shear strength, toughness etc., may be
reasonably improved from those of the individual elements .3-4)
1.2. Al Alloys
In recent few decades, the aluminium alloys have been
used widely in automotive, automobiles, and aerospace
industries. Aluminium is the materials of choice for various
transportation and structural materials and is a cheaper
alternative to steel in few of the cases. For instance, alumin-
ium alloys containing the additives, Mg and Si, are now
being replaces with steel panels in automobiles. This is
particularly true in view of their excellent and attractive
properties, light weight, less fuel consumption, and ease of
transportation. Various elements are available for alloying in
aluminium matrix, most commonly used ones are Cu, Mn,
Mg, Si, Zn, etc. In outer atmosphere, aluminium forms an
inherent surface layer of aluminium oxide which protects it
for external environmental damage. Due to these factors, alu-
minium alloys were the important subject of research in the
past few decades.3-4)
1.3. Nomenclature of Aluminium Alloys
Aluminium alloys have been divided into various categories
depending up to the major alloying element. The designations
have been devised by the Aluminium Association Wrought
Alloy Designation System. The nomenclature system consists
of four numerical digits as shown in Table 1.3)
1.4. Characteristics of Al Alloys
Aluminium alloys covers a wide set of mechanical, physical,
electrical and thermal properties which are improved upon the
pure aluminium metal. The important properties include:
(1) Density: Aluminium is a light weight metal with a
density of about 2.7 g/cm3. If we compare with steel,
aluminum has approximately 1/3 of the density of the steel.
†Corresponding authorE-mail: [email protected]
© 2017, The Korean Microelectronics and Packaging Society
This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License(http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work isproperly cited.
36 Ashutosh Sharma, and Jae Pil Jung
마이크로전자 및 패키징학회지 제24권 제3호 (2017)
(2) Oxidation resistance: As already discussed, aluminium
contains an inherent oxidized layer of alumina over its
surface that prevents it from the progressive oxidation and
corrosion attack when exposed to air.
(3) Electrical and thermal properties: Aluminium alloys are
highly electrically and thermally conductive. For example,
the thermal conductivity of aluminium is approximately
twice to that of copper of similar weight of both elements.
1.5. Al-Si Alloy
Al-Si alloys have a special class among all aluminium alloys.
Their importance in various multidimensional industries lies in
the fact that they exhibit high strength/weight ratio, excellent
wear and oxidation resistance, low density and coefficient of
thermal expansion (CTE) etc. Alloying aluminium with
silicon improves the fluidity and reduces the shrinkage levels
during solidification and processing which improves the
weldability and castability. The silicon in Al-Si alloy plays an
important role in depressing melting point and improving
strength. Silicon has a density of around (2.34 g/cm3) which
further reduces the weight of the resultant cast alloy
component. The poor solubility of Si in Al causes it to
segregate out as a residual hard Si phase and improve the
tribological properties.5) The eutectic point in Al-Si system is
found near 12.6 wt% silicon having a melting point of
≈577oC. The eutectic Al-Si alloy designates the special and
important composition with a minimum possible melting
point of Al-Si alloys as per the phase diagram.2-3, 4-5)
As noted from the Table 1, the Al-Si alloys are marked as
4XXX alloys as per the Aluminium Association Wrought
Alloy Designation System. The main characteristics of the
4XXX series aluminium alloys are:3)
(1) Heat treatable
(2) Excellent fluidity and strength
(3) Compatible to joining by brazing and soldering processes.
1.6. Applications of Al-Si Alloys
(1) In vehicles body parts, trains, structural parts, body
structure and air conditioning. Cast Al alloys have been in
use in various automobile bodies for a long time. In vehicle
engines, the heavy engine blocks are now made from cast
iron to aluminium alloys for weight reduction. Aluminium
castings are almost used for making pistons, piston heads,
axle, housings, and shafts etc. Al alloy castings are also used
for wheels, brackets, brakes, suspension supports, air bag,
shafts, knuckles, housings, instrument panels, etc. Al alloys
have also been used mostly in heat exchangers for joining,
especially Al-Si alloys. Al-Si is used generally for many
automotive joining and body parts, e.g., pistons, cylindrical
linings, etc.6)
(2) As already discussed, Al-Si alloys are the most
diversified foundry alloys for engine pistons in automotive.
However, the hypereutectic alloys are limited in use at
commercial scale because of the difficulty in casting and
machining due to the high Si percentages. Addition of high
percentages of Si in Al, also increases the amount of heat
capacity removed dissipated from the alloy for solidifying in
casting operations.7)
2. Phase Diagram
Al-Si alloy is fundamental binary eutectic phase diagram
with limited solubility of Al in Si and limited solubility of Si
in Al. The only invariant reaction in Al-Si phase diagram is
given by:
L → α + β (eutectic)
Here, the liquid phase is L, The α-phase is primarily Al, and
β is mainly Si. It is now widely recognized that the eutectic
point is around ≈577oC at 12.6% Si. Aluminium silicon alloys
are the casting alloys which are most used in foundry for the
Table 1. Nomenclature of Al Alloys and Applications3)
Alloy Series Major Alloying Element Uses
1XXX Nil (High Purity Al) Electrical Conductors, Chemical Processing
2XXX Cu Aerospace, Automotive
3XXX Mn Architecture
4XXX Si Brazing, Automobiles, Heat Exchangers
5XXX Mg Submarines, Ships
6XXX Mg, Si Architectural Extrusions
7XXX Zn Aircrafts
8XXX Others (Fe, Ti, Ni etc.)
9XXX Not Assigned
Possibility of Al-Si Brazing Alloys for Industrial Microjoining Applications 37
J. Microelectron. Packag. Soc. Vol. 24, No. 3 (2017)
fabrication of piston in automotive engines.3-4, 6-7) The Al-Si
alloys can be categorized into three major categories:
(1) Hypoeutectic (<12.6 wt% Si),
(2) Eutectic (≈12.6 wt%Si), and
(3) Hypereutectic alloys (>12.6%Si, especially 17-23 wt
%Si).
3. Modification in Al-Si Alloys
The variation in silicon shape, size, and structure in Al-Si
alloy can be found in between different sections of cast
structure. This affects the mechanical properties of the Al-Si
specimens drastically and fail the joint. This problem
becomes severe when applied to the brazing industries for
joining applications. Primary Si, therefore, must be modified
by using a number of approaches in the literature for a better
and improved wear and friction resistance. All these
mentioned reasons prohibit the use of hypereutectic Al-Si
alloys over hypoeutectic Al-Si alloys. On the contrary, the use
of hypoeutectic and eutectic Al-Si alloys is widespread due
to their efficient method of production in the absence of big
Si needles, good control of process parameters, and easier
machinability. However, most of them are not suitable for high-
temperature applications, as the tensile strength diminishes at
elevated temperatures. Therefore, modification of Si, for hyper-
eutectic Al-Si alloys, is necessary for uniform properties and
high stability of the alloy.9-12)
3.1. Microstructural Modification by Grain Refiners,
Inoculants, and Nucleating Agents
Hypoeutectic Al-Si alloys have a higher amount of α-Al
in their microstructure. Unmodified Al–Si alloy has big, hard
and brittle flakes of silicon that results in brittle failure during
operation. To prevent this modifiers are added to Al-Si alloys
to modify the Si particles from big needles/plates to fine
round shape. As a consequence, the mechanical properties of
the Al-Si alloys is greatly improved. Grain refiners have been
tried to refine the α-Al grains thereby causing the grain
refinement of α-Al phase in the form of a columnar grain
structure after solidification. Various kinds of Al-Ti master
alloys, e.g., Al-Ti-B alloys have been tried as a grain refiners
in the past for Al-Si alloys.9)
The mechanism of grain refinement can be well understood
by the nucleation and growth process of Al grains. Theoretically,
the process involves both the homogeneous and heterogeneous
nucleation. At industrial level, the wrought and cast alumin-
ium alloys are generally grain refined before actual casting.
Various commercial nucleating agents are also commonly
used. For example, inoculating agents are often added in the
aluminium melt during casting. In last few decades, there
have been many investigations to avoid or stop the grain
growth of the aluminium melts, e.g., Al-Ti, Al-B, Al-Ti-C, Al-
Ti-B, and Al-Sr-B alloys.9, 13-14) The commercially available
grain refining agents are summarized in Table 2.
Fig. 1. Al-Si binary phase diagram.8)
Table 2. Common Grain Refiners in Al-Si Alloys.14)
Alloy Composition
Al-TiAl-10Ti
Al-6Ti
Al-Ti-B
Al-3Ti-1B
Al-5Ti-0.6B
Al-3Ti-0.2B
Al-5Ti-0.2B
Al-10Ti-0.4B
Al-1.6Ti-1.4B
Al-1.2Ti-0.5B
Al-3Ti-3B
Al-1Ti-3B
Al-B
Al-10B
Al-5B
Al-3B
Al-Sr-B Al-10Sr-2B
Al-Sr-Ti-B Al-10Sr-1.6Ti-1.4B
Al-Ti-CAl-6Ti-0.02C
Al-3Ti-0.15C
Al-ScAl-1Sc
Al-2Sc
Mg-ZrMg-25Zr
Mg-33.3Zr
38 Ashutosh Sharma, and Jae Pil Jung
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3.2. Microstructural Modification by Friction Stir Pro-
cessing
Various methods like friction stir processing (FSP) approach
has been tested by few researchers for Si modification.
According to the investigation carried out by Sima Ahmad et
al.,15) while working on A356 alloy consisting of primary α-
Al dendrites and inter dendritic irregular Al–Si eutectic
regions, the use of FSP resulted in a significant modification
of the big and large Si needles and Al-dendrites, thereby
creating a uniform distribution of fine and spherical Si
particles in the Al-Si matrix. The authors explained this
behavior related to the intense plastic deformation at high
friction temperature during FSP that gave rise to the
development of the fine and equiaxed dynamic recrystallized
grains.15) It was also reported that the an increase in the
rotation rate resulted in an decrease of Si size and aspect ratio
as well as the porosity level also found to reduce due to the
severe stirring effect.
3.3. Microstructural Modification by Nanoparticles
In recent few years, there has been an enormous research
activities on refinement of intermetallic compounds by use of
nanoparticles.16-24) In Al alloys, various efforts have been paid to
refine the Si needles as well the CuAl2 phases by nanocomposite
approach.25-26) The various kinds of nanoparticles are used to
refine the Al grain size, Si needles as well the CuAl2
intermetallic in Al-Si-Cu alloys. Sharma et al. have
published recently a number of research results where they
have used ZrO2, SiC, La2O3 to refine the Si needles and
intermetallic compounds in Al-Si-Cu alloys.27-32)
4. Brazing characteristics of various Al-Si alloys
The brazing of Al alloys involves a number of steps
according to the joint material. Prior to brazing, the joint
surfaces should be cleaned to have sufficient wettability and
to prevent the buildup of defects in the brazed joints. The
various types of pre-requisites are as follows:
1. Surface cleaning
2. Optimum clearance and fit
3. Application of flux of good quality (NOCOLOK etc.)
4. Proper fixation of the two parts
5. Application of heat to the joint
6. Finally, cleaning of the joint, in case of any residue left.
In literature, Al-Si filler is most popular among brazing
industries, for joining various parts of heat exchanger,
automotives and aerospace applications. Therefore, the design
of a lower melting brazing filler alloy is a serious concern in
brazing industries. The most common composition of Al-Si
filler is Al-12 wt %Si in a near eutectic composition with a
melting point at 577oC. This melting temperature is in close
proximity of the melting point of various aluminium alloys.
Therefore, we need a low Al-Si melting filler. Various kinds
of Al-Si alloys have been developed by adding elements and
other impurities. A number of alloys are available where
copper have been added.26-28) For example, to lower the
melting point copper is primarily used to form Al-Si-Cu
alloy.29) The microstructure of brazed Al-Cu joint bonded
using Al-Si-Cu filler is shown in Fig. 2. The microstructure
contains the Si particles and big CuAl2 IMC compounds. The
authors have found that CuAl2 are formed at the brazed joint
which are not desirable.
The addition of copper is beneficial to depress the melting
point, and, at the same time, it produces several undesirable
intermetallic compounds like CuAl2. The generation of
CuAl2 is not recommended from the brazing point of view
as it can generate cracks and fail the joint. In another result
the authors have added Zn and Cu in their work to obtain the
melting point around 500oC. The microstructure of the
brazed joint is given in Fig. 3.
Addition of Zn has also been also tried by a number of
researchers in Al-6.5Si-42Zn and Al-6.5Si-42Zn-0.5Sr, with
Fig. 2. Al-Cu brazed joint using Al-Si-Cu filler alloy.
Fig. 3. Al-Cu brazed joint using Al-Cu-Zn filler alloy.
Possibility of Al-Si Brazing Alloys for Industrial Microjoining Applications 39
J. Microelectron. Packag. Soc. Vol. 24, No. 3 (2017)
a significant depression in melting point around 520oC.
However, Zn vaporizes quickly generates pores in the joint.
In another study, Niu et al. investigated on Al-Si-Ge-Zn alloy
and found improved wetting.34) The addition of Ge is not a
good idea because of extremely higher cost around 400 times
than the Al. This makes overall process highly expensive.35)
Other reports also suggest to combine Cu with Ni, e.g., Al-
20Cu-2Ni-5Si with a melting temperature below 538oC and
attractive shear strength of the joint greater than 75 MPa.33)
Recently, nanoparticle reinforced Al-Si alloy have also been
developed for low melting brazing, for example, Al2O3,
ZrO2, La2O3, SiC, etc.19, 27-32) Recently, the brazed joint
microstructures using ZrO2 nanoparticle doped fillers are
given in Fig. 4. The presence of optimum concentration of
nanoparticles (≈0.05 wt%) improves the brazed joint strength
by refining the Si needles and dangerous IMC compounds.29)
However, the research on multidimensional approaches for
soldering and brazing nanofillers is in the beginning stage
and more efforts are needed to finally bring nanocomposite
fillers in the market in coming future.26-28, 36)
5. Conclusions
Alloying elements provide the improved microstructure of
aluminium and decides the final properties of the alloy.
Microstructural modifications are usually brought about by
the precipitates causing an enhancement of the strength and
durability at high temperatures. The addition of copper
triggers the formation of GP zones from the supersaturated
solid solution during aging after heat treatment suitable for
aerospace. Silicon provides mostly fluidity and castability
and suitable for automotive applications. Magnesium too
forms precipitates and increase strengthening of the alloy. Ti
and B improves the grain refinement and hence provide
additional strengthening. Nickel, though expensive used in
minute concentration produces aluminides with copper and
aluminium for high-temperature stability. Addition of Mn
improves the yield strength and ultimate tensile strength,
though a reduction in ductility is also observed. Recently,
various reports have shown a beneficial effect of the addition
of rare earth elements and nano ceramic oxides to refine the
grain size as well as the various IMC and Si morphology.
This will also help to fabricate the nanocomposite fillers at
mass production scale in coming decades. It can be suggested
from this review that an alloying of the aluminium alloys
affects the various properties, therefore the selection of
alloying element should be exercised depending on its role
and suitability for the particular brazing application.
Acknowledgements
This work was supported by the Energy Efficiency &
Resources Core Technology Program of the Korea Institute of
Energy Technology Evaluation and Planning (KETEP) and
was granted financial resource from the Ministry of Trade,
Industry & Energy, Republic of Korea (No. 20142020104380).
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